Extravascular hematomas are characterized by blood extravasation into the body. One form of extravascular hematoma is intramuscular hematoma, i.e. extravasation of blood into muscle tissue affected by trauma, with preserved integrity of the epimysium, e.g. in a limb. Significant limb hematomas occur in diverse populations, from professional athletes to amateur runners and exercise enthusiasts who sustain muscle injuries from repetitive overuse, as well as sharp and blunt limb trauma. The main symptom related to the onset of an intramuscular hematoma is pain, which may be debilitating. Extravascular hematomas are also possible elsewhere in the body, e.g. in and around the internal organs.
There are currently no short-term treatment options for large extravascular hematomas. Drainage of extravascular hematomas is generally difficult if not impossible because even large percutaneously placed drains are inefficient due to the firm gelatinous consistency of the hematomas. Conservative treatment includes rest, ice and compression, and the return to full activity is generally not possible before a period of 10-20 weeks. In addition, posttraumatic myositis ossificans—calcification of muscle—occurs as a complication in approximately 20% of large hematomas. It is responsible for considerable morbidity, with symptoms of prolonged pain, diminished flexibility, local tenderness and stiffness lasting an average of 1.1 years. One of the most devastating sequelae of large intramuscular hematomas is extremity compartment syndrome (ECS), which occurs when muscle tissues take on excess fluid (moderate to large hematoma or muscle swelling due to inflammation) creating pressure that reduces blood flow and ischemic injury. Increased pressures can cause irreversible damage over time, resulting from loss of vascular perfusion leading to loss of limb function/viability, in some cases requiring amputation.
A rapid, definitive intervention aiming at evacuation of the space-occupying hematoma would reduce pain, improve function, and avoid long term sequelae. Ultrasound is known to promote intravascular clot breakdown, as both a standalone procedure and used in conjunction with thrombolytic drugs and/or microbubbles. In-vitro and in-vivo studies have been conducted over the years, and acoustic cavitation is widely accepted as the dominant mechanism for mechanical disruption of the clot integrity and partial or complete recanalization of the vessel. Recently, a technique termed histotripsy that employs high-intensity focused ultrasound (HIFU) has been demonstrated to dissolve large in vitro and in vivo vascular clots without thrombolytic drugs within 1.5-5 minutes into debris 98% of which were smaller than 5 microns. However, this approach cannot be applied to the large extravascular hematomas due to their large volume (20-50 cc's) compared to intravascular clots, which necessitates much higher thrombolysis rates to complete the treatment within clinically relevant times (˜15-20 minutes).